This invention relates to motor drive circuits for electric motors, in particular but not exclusively for use in automotive applications.
It is known to provide electric motors in a range of automotive applications, including electric power assisted steering systems where the motor acts directly upon a part of a steering system to generate a driver assistance torque, and in braking and steering systems where the motor drives a pump which in turn moves hydraulic fluid around the system.
It is known to drive a multiphase electric motor by connecting each phase of the motor to motor drive circuit, typically in the form of a bridge. In a known drive circuit the end of each phase is connected to an upper arm including a switch connected to one terminal of a battery, and a lower are including a semiconductor switch connected to the other terminal of the battery. For a three phase motor the drive stage may comprise a three phase bridge with three of these pairs of arms.
In use, a controller applies voltage patterns to each switch of the drive circuit to cause the switches to open and close rapidly in a defined pattern, thus controlling the amplitude and phase of the voltage applied to each phase of the motor. The phases can be advanced or retarded relative to any back emf voltages generated by the motor rotation as required depending on the motor control strategy used.
With a multi phase motor connected to a drive circuit the motor will generate a back emf which can exceed the battery supply voltage. This may happen when the motor is not being driven by the bridge circuit but is subject to a torque applied to the rotor that causes it to rotate at high speed, or even where it is being driven of the speeds are high enough. This back emf can prove problematic if left unchecked. FIG. 1 illustrates a possible closed path around a motor 100, through the body diodes of the MOSFET switches in a drive bridge 101 and the battery supply 102. When motoring the high back emf may be overcome by advancing the current waveform applied to the motor to weaken the magnetic field prior to the peak back emf. This allows the motor to spin faster than would otherwise be possible.
In the event of a fault in the drive circuit, or in the supply lines connecting the circuit to the power supply, or with the motor controller, or simply when the motor is not being driven but is being back-driven by a torque applied to the rotor causing it to act as a generator, it may not be possible to sufficiently weaken the field by advancing the electric field. This will result in the full back emf being generated which is then conducted via the MOSFET diodes onto the battery—notionally onto the DC battery bus and from there back into the battery. While not destructive, the applicant has appreciated that this can act as a significant drag on the motor, limiting high speed operation.
For many applications a rising demand is being seen in providing additional safety through the use of redundancy in the circuit design. In one arrangement, a dual drive bridge circuit is proposed with either two motors or one motor having two sets of independent phase windings and the bridge split into two independent lanes, each lane comprising a full H-bridge circuit which is connected to one of the sets of phases. In use, one of the lanes can be used to drive the motor whilst the other is kept in reserve for use where the one lane is faulty. Alternatively, both can be driven at the same time and one turned off when a fault in that lane is detected. The benefit of driving both at the same time is that the power losses across both lanes can be lower that of a single bridge driving, allowing thinner wires and lower rated switches to be used.
In a fully independent dual lane arrangement, i.e. dual bridges, each lane may receive control signals from its own control circuit and may have its own independent connections to the battery. Alternatively, for some applications a shared connection to the battery is provided, and a shared controller.
Where a dual lane arrangement is provided, and one lane is at fault, a back emf generated in this lane could create drag that opposes the attempts of the good lane to drive the motor. This would reduce the peak motor speed available.